Bottom Line:
van der Waals (vdW) forces play a fundamental role in the structure and behavior of diverse systems.Nevertheless, they do not influence the spatial configuration of the adsorbed molecules.Their effect on the electronic density is a nonisotropic, delocalized accumulation of charge between the molecule and the slab.

ABSTRACTvan der Waals (vdW) forces play a fundamental role in the structure and behavior of diverse systems. Because of development of functionals that include nonlocal correlation, it is possible to study the effects of vdW interactions in systems of industrial and tribological interest. Here we simulated within the framework of density functional theory (DFT) the adsorption of isooctane (2,2,4-trimethylpentane) and ethanol on an Fe(100) surface, employing various exchange-correlation functionals to take vdW forces into account. In particular, this paper discusses the effect of vdW forces on the magnitude of adsorption energies, equilibrium geometries, and their role in the binding mechanism. According to our calculations, vdW interactions increase the adsorption energies and reduce the equilibrium distances. Nevertheless, they do not influence the spatial configuration of the adsorbed molecules. Their effect on the electronic density is a nonisotropic, delocalized accumulation of charge between the molecule and the slab. In conclusion, vdW forces are essential for the adsorption of isooctane and ethanol on a bcc Fe(100) surface.

fig5: Charge density difference(ρdifff) of ethanoladsorbed on the bcc Fe(100) surface at d = 2.00 Å.The charge density difference is defined and plotted in an analogousway to Figure 4.

Mentions:
In addition to dispersion forces,a weak electrostatic interactioncontributes to the adsorption of ethanol on Fe(100). According toa comparison between the results obtained with revPBE+LDA and vdW-DF,the amount of charge that accumulates as a consequence of nonlocalinteractions is larger and extends over a wider area (Figure 5). However, in contrast to the case of isooctane,in this system the charge redistributions predicted by these functionalsdiffer considerably. According to the results obtained with the revPBE+LDAfunctional, in the absence of nonlocal correlations the Pauli repulsionproduces a region of deficit of charge between the surface and themolecule and displaces the electronic density above the molecule.This considerable redistribution of charge (Figure 5a) and the small adsorption energy (Table 1) calculated with this functional suggest that it is overlyrepulsive in this system. A similar behavior has been observed beforefor the revPBE functional (exchange and correlation) in moleculesat short separations.75 A calculation withthe revPBE functional predicts a larger absorption energy (Figure 8), but it is nevertheless less than half of theone calculated with the PBE functional. Evidently, this system isheavily influenced by the description of the exchange energy, sincethe only difference between these two functional lies in the exchangeterm. The charge redistribution calculated with the PBE functional(Figure 6a) resembles the one calculated withvdW functionals (Figures 5b and 6b). Since PBE does not include the effects of nonlocal correlations,this last result indicates that an important contribution to the bindingbetween the ethanol and the Fe(100) surface cannot be attributed tothe dispersion forces alone. On the basis of previously estimatedadsorption energies, it has been proposed that a weak chemisorptionis involved in the binding mechanism.56 Our calculations, however, do not show a charge transfer large enoughto consider the formation of an ionic bond, and the analyses of theelectron localization function (ELF)76,77 do not pointto the existence of localized electrons forming a bond between themolecule and the metallic slab (Figure 7). Furthermore, it has been shown that the oxygen statesare almost unperturbed,56 since they donot exhibit a strong hybridization with the d states of the bindingFe atom. The optB86b-vdW functional predicts a lesser amount of chargebetween the ethanol and the iron slab than the one calculated withPBE, particularly between the hydroxyl group and the closest ironatom to it (Figure 6c). Since this effect isaccompanied by an increase in the adsorption energy, a weak electrostaticinteraction together with the dispersion forces may constitute themain mechanisms contributing to the adsorption. Nevertheless, thiscomparison does not allow separation of these contributions, sincethese functionals differ in the exchange term.

fig5: Charge density difference(ρdifff) of ethanoladsorbed on the bcc Fe(100) surface at d = 2.00 Å.The charge density difference is defined and plotted in an analogousway to Figure 4.

Mentions:
In addition to dispersion forces,a weak electrostatic interactioncontributes to the adsorption of ethanol on Fe(100). According toa comparison between the results obtained with revPBE+LDA and vdW-DF,the amount of charge that accumulates as a consequence of nonlocalinteractions is larger and extends over a wider area (Figure 5). However, in contrast to the case of isooctane,in this system the charge redistributions predicted by these functionalsdiffer considerably. According to the results obtained with the revPBE+LDAfunctional, in the absence of nonlocal correlations the Pauli repulsionproduces a region of deficit of charge between the surface and themolecule and displaces the electronic density above the molecule.This considerable redistribution of charge (Figure 5a) and the small adsorption energy (Table 1) calculated with this functional suggest that it is overlyrepulsive in this system. A similar behavior has been observed beforefor the revPBE functional (exchange and correlation) in moleculesat short separations.75 A calculation withthe revPBE functional predicts a larger absorption energy (Figure 8), but it is nevertheless less than half of theone calculated with the PBE functional. Evidently, this system isheavily influenced by the description of the exchange energy, sincethe only difference between these two functional lies in the exchangeterm. The charge redistribution calculated with the PBE functional(Figure 6a) resembles the one calculated withvdW functionals (Figures 5b and 6b). Since PBE does not include the effects of nonlocal correlations,this last result indicates that an important contribution to the bindingbetween the ethanol and the Fe(100) surface cannot be attributed tothe dispersion forces alone. On the basis of previously estimatedadsorption energies, it has been proposed that a weak chemisorptionis involved in the binding mechanism.56 Our calculations, however, do not show a charge transfer large enoughto consider the formation of an ionic bond, and the analyses of theelectron localization function (ELF)76,77 do not pointto the existence of localized electrons forming a bond between themolecule and the metallic slab (Figure 7). Furthermore, it has been shown that the oxygen statesare almost unperturbed,56 since they donot exhibit a strong hybridization with the d states of the bindingFe atom. The optB86b-vdW functional predicts a lesser amount of chargebetween the ethanol and the iron slab than the one calculated withPBE, particularly between the hydroxyl group and the closest ironatom to it (Figure 6c). Since this effect isaccompanied by an increase in the adsorption energy, a weak electrostaticinteraction together with the dispersion forces may constitute themain mechanisms contributing to the adsorption. Nevertheless, thiscomparison does not allow separation of these contributions, sincethese functionals differ in the exchange term.

Bottom Line:
van der Waals (vdW) forces play a fundamental role in the structure and behavior of diverse systems.Nevertheless, they do not influence the spatial configuration of the adsorbed molecules.Their effect on the electronic density is a nonisotropic, delocalized accumulation of charge between the molecule and the slab.

ABSTRACTvan der Waals (vdW) forces play a fundamental role in the structure and behavior of diverse systems. Because of development of functionals that include nonlocal correlation, it is possible to study the effects of vdW interactions in systems of industrial and tribological interest. Here we simulated within the framework of density functional theory (DFT) the adsorption of isooctane (2,2,4-trimethylpentane) and ethanol on an Fe(100) surface, employing various exchange-correlation functionals to take vdW forces into account. In particular, this paper discusses the effect of vdW forces on the magnitude of adsorption energies, equilibrium geometries, and their role in the binding mechanism. According to our calculations, vdW interactions increase the adsorption energies and reduce the equilibrium distances. Nevertheless, they do not influence the spatial configuration of the adsorbed molecules. Their effect on the electronic density is a nonisotropic, delocalized accumulation of charge between the molecule and the slab. In conclusion, vdW forces are essential for the adsorption of isooctane and ethanol on a bcc Fe(100) surface.